A SMPS such as a DC-DC converter includes a switch element that is switched to convert a power source to a regulated DC output voltage. The DC-DC converter may have a flyback quasi-resonant (QR) topology that includes a resonant circuit. The resonant circuit may include a parasitic capacitance of the switch element and an inductance of a winding of a transformer.
In QR switching, energy is stored in the transformer during a charging phase when the switch element is turned on. The energy stored in the transformer is released in a discharge phase when the switch element is turned off.
After the energy stored in the transformer is dissipated (that is, when the discharge phase ends), the resonant circuit causes a node voltage of the switch element to ring. In a technique known as valley switching, the switch element is turned on to begin a next charging phase in response to the resonant ring reaching a low level, that is, in the valley of the resonant ring.
To prevent the switch element from inadvertently turning on, the switch element is prevented from being turned on during a blanking time. The blanking time may be controlled according to an indication of a load current to improve the efficiency of the DC-DC converter. The relationship between the load current and the blanking time is called a foldback curve. The foldback curve can be expressed as a relationship between a feedback voltage corresponding to the load current and a frequency corresponding to the inverse of the blanking time.
To further improve the efficiency of the DC-DC converter, a plurality of foldback curves respectively corresponding to a plurality of operating conditions may be employed. For example, a first foldback curve may be used when an input voltage to the DC-to-DC converter (herein referred to as a line voltage) is high, and a second foldback curve may be used when the line voltage is low.
To improve the efficiency of a DC-DC converter under light load conditions, the DC-DC converter may enter a burst mode. In an illustrative burst mode, wherein a feedback voltage decreases as the output voltage increases and increases as the output voltage decreases, switching of the switching element stops in response to the feedback voltage dropping below a first threshold, and does not resume until the feedback voltage rises above a second threshold higher than the first threshold.